Studies of the role of tumor specific antibodies as promoters or inhibitors of tumor growth, and of antibodies as possible therapeutic agents in treating human cancer, have been handicapped by the lack of methods for preparing such antibodies in a pure form. These investigations are designed to prepare tumor specific 125I-labeled antibody to carcinogen induced rat tumors with the following characteristics: (a) In vitro it will bind quantitatively to the outer membranes of living tumor cells of the tumors used for immunization and not to normal tissue cells or normal cellular antigens; (b) Injected intravenously it will show preferential tumor localization in tumors growing in the inbred strain in which the tumor originated. It is postulated that the relative lack of success of many previous attempts to prepare radioactive tumor localizing antibody is because such antibody was almost always obtained by xenogeneic immunization, and the resulting antibodies cross reacted with antigens present on normal tissues. Therefore, the antibodies used in these studies will be induced by allo- or isogeneic immunization. Antibodies will be isolated from the gamma-globulin fraction of immune sera by absorption on and elution from membrane fractions prepared from tumor cells or from preparations of viable single cell tumor suspensions. Alloantibodies not specific for tumor, and autoantibodies reacting with normal tissues will be removed by absorption with cells or cell membrane fractions prepared from normal rat tissues or, in preliminary studies, by in vivo absorption in normal rats.